Totally Tubular!

Well, that phrase sure dates me. Gag me with a spoon... We'll start this with another box on wheels in basic black.

Imagination has to work a little harder to make this out to be a tube on wheels instead of a box, but the little box on top of the big box helps. Not everything in real life is a box with flat sides. What we need is a way to portray an object's shape. Fortunately we can do that with shading.

Since this car is black, we'll be working in shades of gray. Pick a middle gray (I used 40% black) and draw a horizontal line about 1/3 of the way down from the top of the tank.

This next step might seem a bit technical. We need an smooth progression from the value of this lighter stripe to the darkest value (black, in this case). Graphics programs may vary but you can usually use a tool such as an eyedropper to find the RGB values of any color in your drawing. You can also manually set the RGB colors to any value you want. Since we are working with grays, each component will have the same value. For instance, the black has an RGB value of R=0, G=0 and B=0. The lighter stripe is 153-153-153.

To make even gradations from the stripe to the black of the shadow on the bottom of the car, count the rows of pixels and divide that into the difference of the color values. The difference in value here is 153 - 0, or 153. Since there are 8 rows, we divide 153 by 8, which is about 19. Paint each row with a value that is 19 less than the one above it. The values you will use are 153, 134, 115, 96, 77, 58, 39, 20, 0.

If you don't want to mess with the math, just eyeball the shades of gray, but beware - if the change is not even it may look like flat spots on the side of your tank. Which is good to know, since some tank cars actually have flat spots on their sides.

Notice how the smaller version is much more convincing than the larger one.

Now paint the rows above your initial stripe using the same values in reverse order.

Finish it off by using the same technique to round out the dome. I also knocked off the corners of the tank to make it look less boxy. To whet your appetite for more advanced techniques, I added a highlight by laying down some white with varying degrees of transparency. If your graphics program has this capability, learn how to use it for great lighting and weathering effects.